1. Field of the Invention
The invention relates in accordance with a method for reducing the water and energy consumption of a paper machine with the help of a vacuum system and optimization of solids content. Furthermore, the invention relates to the use of an apparatus and a hybrid vacuum system.
2. Description of Background Art
As is generally known, a papermaking process is highly energy-intensive. The greatest energy consumers can be basically listed as the heating of raw materials, pump units at the wet end and the dryer section itself.
In the heating of raw materials for papermaking, fiber and fillers are taken to the paper mill from outdoors storage. Hence, they must be heated to the process temperature of 40-50 degrees centigrade The stock is mixed with water, whose amount typically by weight is 10 to 20 times greater than the amount of wood fiber and filler materials in the resulting production furnish (furnish consistency in the headbox is generally 0.3-1 percent, max. 2 percent, which translates into 1 part of fiber and similar solids in 99-300 parts of water). This water volume is recirculated in the process even if the process does not have a closed circulation system. Water consumption per produced ton of paper is about 10-20 m3/t. The necessary amount of water is generally taken from a lake/river at a temperature of 0-25 degrees centigrade, depending on the season of the year. Today, the process temperature is elevated up to 40-50 degrees centigrade in order to improve dewatering drainage. Hence, the effluent water passed from the mill to wastewater treatment contains a great amount of low-value thermal energy whose recovery concurrently is unprofitable or complicated.
Additionally, papermaking needs a lot of air that also must be heated. Many mills use large volumes of hot air which is exhausted from the process without being utilized by heat recovery.
The above-mentioned great amount of circulation water requires massive pumping power that is another major energy consumer. Eventually, this energy is converted into heat thus providing a major portion of the thermal energy required by the process. Drying the paper web on the wire and press sections is partially implemented with the help of vacuum. This portion of the process is known as the process vacuum system. Also the vacuum system is a prominent energy user that typically consumes 20 percent of the mill's overall power.
After passing through the wire and press sections, the solids content of the paper web is generally in the range of 40-50 percent. The final moisture content of the paper web is 6-8 percent. Drying the web after the press takes place by evaporation. Evaporation of water is a highly intensive consumer of energy that is obtained from steam. Hence, the solids content of the paper web is advantageously maximized upstream of the dryer section in order to minimize steam consumption.
As said, an important portion of a paper machine is its vacuum system. The vacuum systems of a paper machine can be divided into two basic arrangements: a water ring pump system and a turboblower system.
In FIG. 1 is illustrated a typical paper machine equipped with water ring pumps. The drawing also shows conventional dewatering equipment of a paper machine that will be discussed later in the text. The system generally comprises pumps with 6-15 pieces that serve the locations of the paper machine 5 requiring vacuum at different sub-atmospheric pressure levels. In a water ring pump 4, the rotating water acts as a piston that compresses the entering gas. The compression takes place isothermally, whereby the thermal energy released from compression is mainly absorbed by the seal water passed into the pump 4. A great amount of seal water is required, 100-400 l/min per pump, but 80-90 percent thereof can be recirculated if elevation of water temperature can be prevented through cooling the liquid circulation. However, there will be lost the thermal energy recoverable from the compression cycle, and also from a portion of the heat released in the process. As the spent seal water contains fiber and impurities, it is generally passed out from the mill's water circulation. The overall efficiency of a water ring pump 4 is dependent on the vacuum level and pump rotation speed. Efficiency at a high vacuum is better than at low vacuum.
Alternatively, the vacuum system in paper machines can be implemented with centrifugal blowers of the type generally known as turboblower. The vacuum locations in a paper machine are generally operated at a high vacuum in excess of 60 kPa. Hereby one or two multistage blowers are required to reach the highest vacuum levels. The number of stages, or impellers, arranged in series is typically four pieces. Generally, at each impeller is arranged an intermediate port for connection to lower vacuum locations. The capacity of this blower type cannot be varied by changing the speed of rotation. The only possible way of adjustment is by throttling the air flow. In terms of energy efficiency, this kind of capacity control is uneconomical. Additionally, the system also has at least one single-stage blower for medium vacuum locations. As the exhaust air from the blower system is hot, its thermal energy can be recovered by heat exchangers.
In FIG. 2 is illustrated a typical turboblower system. The pumping efficiency of a turboblower system is slightly better than that of a water ring pump 4. On the other hand, when the system has a smaller number of blowers, the flows to the vacuum locations must be controlled with throttle valves, whereby the overall efficiency of the system is impaired. The greatest benefit is appreciated therein that the blower has no rotating water ring, i.e., it does not need seal water at all. In contrast, a significant problem arises from cost of the system due to the multistage blowers, whose higher price of acquisition and installation together with their auxiliary equipment increase the investment costs.
In addition to the above systems, the dewatering equipment of a paper machine is a vital element. On the wire section 12, water is initially removed from the web by gravitation and with foil effects and centrifugally. Thereupon more differential pressure must be applied across the web, since a major portion of water is transferred from the paper web to the dewatering equipment by compression. Typically, downstream of the felt water level are arranged flat suction boxes 17 having a vacuum therein. At the end of the wire section 12 is generally located a suction roll. In FIG. 3 is shown the conventional construction and operating principle of flat suction boxes 17 and wire suction roll 3.
The vacuum system produces the vacuum in the flat suction boxes and the wire suction roll. Air flows into the suction boxes 17 through the web, whereby a pressure differential is established. This function is highly energy-intensive. The pressure differential causes friction between the suction box 17 and wire section 12, thus increasing the energy consumption of the wire drive motors. Water is removed from the paper web partially along with the air flow, but also due to caliper compression of the web, particularly for thicker paper grades having a basis weight greater than 80 g/m2.
The wire suction roll 3 has holes 15 drilled thereto for suction of air through the web. The passing-through air does not retard the web travel but requires more capacity from the vacuum pumps due to the elevated vacuum level and additional air sucked through the drilled holes 15. Water is collected from the web through the wire into the holes 15 of the roll 3, wherefrom it is ejected centrifugally to a water collection pan adapted about the roll. In practice has been found that no water will pass through the holes 15 of the suction roll 3 if the web speed exceeds 200 m/min. Another practical experience relates to the crucial role of the pan as all water ejected from the roll 3 must be collected into the pan, wherefrom it is returned to the water circulation of the machine. Since the water tends to adhere to the surface of the suction roll 3 and holes 15 due to surface forces, it must be separated with the help of different doctoring arrangements.
It must be noted that while no fresh water is needed for the suction box 17, the wire suction roll 3 consumes water by about 100-200 l/min as lubrication for the seal strips of the vacuum chamber 14 of the wire suction roll 3.
Typical solids content after the wire is 15-20 percent. Solids content is preferably maximized in order to minimize energy consumption at the later discussed press and dryer sections, simultaneously achieving improved machine runnability. A general rule of thumb in the art is that in paper webs a change of 1 percent downstream of the wire section 12 results in 0.25 percent increase in solids downstream of the press section.
In the appended FIG. 4 are shown the results of a study performed by Ph.D. Raeisaenen on the effect of vacuum level and dwell time on the solids content downstream of the wire section. The diagram is reprinted from the annual report 1994 of research program Sustainable Paper (Kestaevae Paperi). As has also found in practice, the diagram clearly indicates the optimum running condition, namely, that the vacuum level must increase toward the trailing end of the wire travel. Another important aspect relates to the maximum solids content attainable at a given pressure differential. As a longer dwell time cannot offer a higher solids content, a higher vacuum level must be applied. Since a vacuum level of about 70 kPa is a practical maximum, compression must be applied to the web downstream of the wire section 12 in order to reach a higher solids content. Down-stream of the wire section on the press section, this function is implemented with the help of presses that remove water from the web to felts and/or holes 15 of the suction roll 3 and/or grooves of a grooved roll. In some paper machines, a press and endless felt are adapted above a suction roll. This arrangement typically achieves a solids content of about 24 percent downstream of the wire section 12.
As elucidated above, running a paper machine involves an extremely great number of factors affecting water and energy consumption. In prior-art arrangements the goal has been to solve one problem at a time. Now the present invention attempts to find a solution by way of examining all the different factors separately and then combining their effect in the overall performance. This approach proved that in the art there still are substantial possibilities of improvement in the various properties of a paper machine. The essential feature of the invention is particularly a reduced consumption of water and energy by virtue of an improved vacuum system and optimization of web solids content. Resultingly, the invention offers reduced energy consumption in a paper machine through increased solids content downstream of the wire and press section and, further, by reducing raw water consumption in the mill. This goal is attained by utilizing conventional equipment in an entirely novel and innovative fashion and enhancing the operating practices of the paper machine 5. In addition to the economic benefit resulting therefrom, the invention facilitates reduced investment costs with regard to the present situation.
The essential features of the present invention are crucial factors in the arrangement defined in the claims. The present invention provides plural significant benefits, while simultaneously avoiding the problems hampering the prior art as discussed above.
The arrangement according to the invention implemented using a so-called hybrid vacuum system, whereby a significant improvement is achieved in the energy consumption of the paper machine in its front end, on its wire section through reduced water and electricity consumption and increased web solids content. A significant feature is that the invention aims to provide a comprehensive improvement of energy consumption rather than simply attempting enhanced energy use in a single component such as a pump.